It is often desirable to determine a battery's state of charge (SOC) for diagnostic functions and development of satisfactory charging regimes. The SOC is expressed as a percentage of the total energy, usually expressed in amp-hours (Ah), storable in the battery when the battery is fully charged. For example, a battery capable of storing 100 Ah when it is fully charged will have 75 Ah remaining when the SOC is 75%.
A conventional method for determining the SOC of a battery, such as a lead-acid battery, uses a measurement of an open circuit voltage (OCV) as a state of charge indicator. Generally, the OCV refers to the potential difference across the battery terminals with no load on the battery. As the battery discharges, the potential difference between the terminals drops. Using this method, conventional lead-acid battery analyzers measure the decrease in OCV from a full charge OCV to determine the battery's SOC.
The OCV of a battery is also related to a specific gravity (SG) of the battery's electrolyte. Specific gravity of the battery refers to a ratio of the density of the battery's electrolyte to the density of water. For example, a flooded cell lead-acid battery with six cells and a full charge specific gravity of 1.28 typically has a full charge OCV of 12.7 volts (V). Problems arise because conventional methods and apparatus do not take into account recently introduced lead-acid batteries having a sulfuric acid electrolyte with a full charge specific gravity of 1.30. These higher specific gravity lead-acid batteries generally have a full charge OCV of 13 V. Because the conventional methods and apparatus for determining SOC cannot distinguish between the lower specific gravity batteries (1.28 SG) and the higher specific gravity batteries (1.30 SG), incorrect SOC determination results when using the higher specific gravity batteries with conventional methods and apparatus.
Thus, there is a need to overcome these and other problems of the prior art and to provide better methodology and apparatus to determine battery SOC.